Tellurite glasses and optical components

ABSTRACT

A family of alkali-tungsten-tellurite glasses that consist essentially of, as calculated in mole percent, 10-90% TeO2, at least 5% WO3 and at least 0.5% R2O where R is Li, Na, K, Cs, Tl and mixtures, that may contain a lanthanide oxide as a dopant, in particular erbium oxide, and that, when so doped, is characterized by a fluorescent emission spectrum having a relatively broad FWHM value.

FIELD OF THE INVENTION

Alkali-tungsten-tellurite glasses and their use in telecommunicationsystem components.

BACKGROUND OF THE INVENTION

Optical components, in particular components employing optical fibersdoped with rare earth metal ions, are widely used in telecommunicationsystems. A major application is in signal amplifiers which employfluorescent ion emission for amplification of a signal. The ion emissionoccurs within the same operating wavelength region as the signals. Pumpenergy excites the rare earth metal ion causing it to fluoresce andthereby provide optical gain.

Glasses, doped with a rare earth metal ion and pumped with appropriateenergy, exhibit a characteristic, fluorescence intensity peak. Theevolution of telecommunication systems, with their increasing demandsfor bandwidth, has created a need for a rare-earth-doped, amplifiermaterial having the broadest possible emission spectrum in thewavelength region of interest. It is a purpose of the present inventionto meet this need.

The bandwidth of a fluorescent intensity curve is, rather arbitrarily,taken as the full width half maximum (FWHM) of the curve in nanometerswavelength. This value is the lateral width of the curve at one half themaximum intensity, that is, at one half the vertical height of the peakof the curve. Unfortunately, many glasses, that exhibit a fluorescencein an appropriate region, exhibit a rather narrow bandwidth. It is afurther purpose of the invention to provide a family of glasses thatexhibit a relatively broad bandwidth.

It is well known that glasses doped with erbium can be caused to emitfluorescence in the 1520-1560 nm. region. This enables a signaloperating in this wavelength range to be amplified. The significance ofthe 1550 wavelength in optical communication has led to extensivestudies regarding the behavior of erbium as a rare earth metal dopant inglasses. It has also led to the study of a variety of glasses as thehost for the erbium ion.

The low phonon energy of tellurite glasses can lead to long emissionlifetimes at certain pump wavelengths. As an example, erbium intellurite glasses exhibits long tau-32 (980 emission) lifetimes relativeto silicates. The long emission lifetimes at the pump wavelength canreduce the efficiency of an amplifier or laser because of excited stateabsorption. A practical 980 pumping scheme can be obtained by co-dopingglasses with components having phonon overtones that are resonant withthe energy difference between 980 and 1530 nm. Such components includeH₂O, B₂O₃, P₂O₅.

For certain applications, long, erbium, tau-32 emission lifetime isdesirable. These include long-band amplifiers, where pumping directlyinto the ground-state absorption does not impact noise figure; alsotilt-free amplifiers, in which 980 and 1480 pump lasers are combined todynamically adjust the gain without affecting the shape of the gainspectrum.

It is also known that glasses with moderately low maximum phononenergies, when doped with thulium, can display fluorescence in thevicinity of 1450 nm. Although this wavelength lies outside of thecurrently used telecommunications band, it still lies within thetransparency window of most commercial optical fiber. With theever-increasing demand for useful bandwidth, there will be a need foradditional amplifier devices that operate over the remaining portions ofthis window that are not covered by erbium.

It is known, for example, that, as the concentration of a rare earthmetal ion, such as erbium, is increased, the optical gain increases upto a certain point. Beyond this point, the fluorescent signal isquenched, and the optical gain decreases. This phenomenon is consideredto result from the dopant, rare earth metal ions interacting with eachother in a manner commonly referred to as clustering. It is anotherpurpose of the invention to provide a family of glasses which is readilycapable of dissolving erbium ions, and which exhibits a broad bandwidthindicating that clustering is inhibited.

It has been reported that certain tellurite glasses, doped with erbiumions, provide a very broad, erbium emission band in the 1540 nm. regionof the spectrum. Glass compositions were not reported, but other studiesindicate that the glasses are alkali-alkaline earth-zinc-telluriteglasses.

Most tellurite glass families require the presence of at least about 50%TeO₂ in the glass compositions to permit glass formation. It has beenthought that a broader range of the glass forming oxide, TeO₂, shouldprovide a variety of structural motifs of the network-forming TeO_(x)species. Such species include polymerized, bipyramidal TeO₄ groups athigh TeO₂ content and isolated pyramidal TeO₃ groups at low TeO₂content. The diversity of species should yield a greater diversity ofstructural sites for the incorporation of dopant ions, such as erbiumions. This avoids the ions clustering and becoming ineffective forfluorescent emission and consequent amplification. Also, the diversityof dopant sites should give rise to broadened emission spectra.

It is, then, a basic purpose of the present invention to provide afamily of tellurite glasses that can be melted over an extensivecompositional region, and in which compounds of rare earth metals, suchas erbium, are readily soluble.

SUMMARY OF THE INVENTION

The invention resides, in part, in a family of alkali-tungsten-telluriteglasses that consist essentially of, as calculated in mole percent,10-90% TeO₂, at least 5% WO₃, and at least 5% R₂O, where R is Li, Na, K,Rb, Cs, Tl and mixtures.

The invention further resides in an optical component for atelecommunication system that is composed of analkali-tungsten-tellurite glass that has a high thermal stability(T_(x)−T_(g)), that readily dissolves rare earth metal oxides and thathas a composition consisting essentially of, as calculated in molepercent, 15-85% TeO₂, 5-55% WO₃, 0.5-40% R₂O, where R is Li, Na, K, Rb,Cs, Tl and mixtures, and 0.005-10% of an oxide in the lanthanide series.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a graphical representation comparing the emission spectrum ofa glass of the present invention and that of a corresponding priorglass,

FIG. 2 is a graphical representation illustrating the thermal stabilityof a glass in accordance with the invention.

FIG. 3 is a graphical representation illustrating the effect of certainco-dopants on fluorescence lifetimes of erbium ions in the presentglasses.

DESCRIPTION OF THE INVENTION

It has been indicated earlier that known tellurite glass familiesgenerally require at least 50 mole percent TeO₂ in their composition.The present invention is based on discovery of analkali-tungsten-tellurite glass family that has a much greatercompositional region. Further, the glasses in this family are readilycapable of dissolving rare earth metals as dopants. In particular, whenthese glasses are doped with an erbium compound, they exhibit broademission features, that is, they exhibit a relatively large FWHM. Thisis especially significant for the commercially important 1.54 μm band.In addition, they provide other features that enhance their ability inproducing optical materials, particularly fibers.

The alkali-tungsten-tellurite glasses are unique in that their TeO₂content may range broadly from 10-90 mole percent. This broad range ofpossible TeO₂ contents offers the possibility of a concomitant varietyof structural motifs of the network-forming TeO₂ species. As indicatedearlier, these include polymerized, bipyramidal, TeO₄ groups when theglass has a high TeO₂ content. With a lower TeO₂ content, isolatedpyramidal TeO₃ groups become possible. This variety of structural motifsyields a greater diversity of structural sites for incorporating dopantions such as erbium ions (Er⁺³). The consequence of this is an enhancedsolution of effective erbium ions and broadened erbium emission spectra.

TABLE I sets forth the compositions in mole percent of representativealkali-tungsten-tellurite glasses in accordance with the invention. Alsopresented are FWHM values in nanometers and thermal stability indexvalues for glasses having these compositions. For comparison purposes, aglass composition, having no tungsten content, is shown as example 5.

TABLE I 1 2 3 4 5 Na₂O 15 10 5 13 12 K₂O 15 10 5 13 12 WO₃ 45 30 15 39 —TeO₂ 25 50 75 35 76 Er₂O₃ 0.05 0.05 0.05 0.05 0.05 FWHM 50 50 58 34 (nm)T_(x)—T_(g)(° C.) 127 134 143 142 108

These glasses may be utilized in bulk form for planar devices. However,it is contemplated that they will also be fabricated in the form ofoptical fibers. For the latter purpose, the thermal stability of theglass is an important consideration. Accordingly, a large value for thethermal stability index (T_(x)−T_(g)) is significant.

The thermal stability index is the temperature interval between atemperature (T_(x)) and the glass transition temperature (T_(g)). Asused here, T_(x) represents the temperature at the onset of crystalformation. Any development of crystals in the glass can be detrimental.

For drawing of fibers, an index value of at least 100° C. is desired.Such an index value, in view of the steepness of the viscosity curve forthese glasses, is sufficient to permit fiberization of the glasses in apractical manner for example, by redrawing “rod-in-tube” preforms.

The property features of the inventive glasses are further describedwith reference to FIGS. 1 and 2.

FIG. 1 is a graphical representation comparing the emission spectrum ofa tungsten-containing, tellurite glass with the emission spectrum of atungsten-free, alkali tellurite glass. Relative emission intensity isplotted on the vertical axis in arbitrary units (a.u.) for comparisonpurposes. Wavelength in nanometers, is plotted on the horizontal axis.

Curve A represents the emission spectrum for a known alkali telluriteglass that contains no tungsten content in its composition. This glasshas the composition of Example 5 in TABLE I. Curve B represents theemission spectrum for a typical, alkali-tungsten-tellurite glass of thepresent invention (Example 3). The dashed, lateral lines indicate theFWHM value for each glass. The flatter and broader character of Curve B,and the consequently greater FWHM value of the tungsten-containingglass, are apparent. This is a typical and significant feature of thepresent glasses.

FIG. 2, also a graphical representation, illustrates the thermalstability in a typical alkali-tungsten-tellurite glass. In FIG. 2,temperature in °C. is plotted on the vertical axis, and TeO₂ content, inmole percent, in alkali-tungsten-tellurite glasses is plotted on thehorizontal axis.

Curve C represents the T_(x) temperatures in °C. for glasses having TeO₂content varying from 20-80 mole percent. Curve D represents the glasstransition temperatures (T_(g)) for the same glasses over the same TeO₂content range.

It will be observed that the T_(g) curve (Curve D), rises slowly toabout 60% TeO₂ and then gradually descends a bit. The T_(x) curve (CurveC) also rises to about 40% TeO₂ content; then levels off.

The significant feature is the vertical distance between the curves atany given content of TeO₂. It will be observed that this verticaldifference (T_(x)−T_(g)) is about 125° C. at about 20% TeO₂ and grows tonearly 150° C. at high TeO₂ content. As indicated earlier, this isextremely important in avoiding devitrification problems during thedrawing of fibers by any preform redraw technique.

In general, the basic glass family of the invention, that is, theundoped alkali-tungsten-tellurite glass family, consists essentially of,in mole percent, 10-90% TeO₂ and the remainder at least 0.005% R₂O,where R is Li, Na, K, Rb, Cs and/or Tl, and at least 5% WO₃. Within thisbroad composition range, a somewhat narrower range has characteristicsthat make the glasses particularly useful in the fabrication of cladfibers doped with erbium for amplification components. The base glasscompositions for this purpose, consist essentially of, as calculated inmole percent, 15-85% TeO₂, 5-55% WO₃, and 0.5-40% R₂O, where R indicatesthe elements recited above. In addition, the base glass will contain0.005-10% of an oxide of a rare earth metal in the lanthanide series,including erbium.

The base glass compositions may be modified to alter the physicalproperties of glasses having these compositions. In particular, they maybe modified for the purpose of providing combinations of core andcladding glasses for an optical fiber. For that purpose, the glassesmust exhibit a difference in refractive indices as is well known.Otherwise, however, it is desirable that the core and cladding glasseshave properties as near identical as possible.

The modifying oxides may include 0-30% MO where M is Mg, Ca, Sr, Ba, Zn,Cd, Pb, Y, La, Gd, Lu, Ti, Zr, Hf, Nb, Ta, Bi, H, B and/or P.

More particularly, glass compositions may be modified by the addition of0-30% MO wherein M is Mg, Ca, Sr, Ba, Zn and/or Pb, 0-20% Y₂O₃ and/orSb₂O₃, and/or 0-15% TiO₂, Nb₂O₅ and/or Ta₂O₅. These additions areparticularly useful in developing core/cladding compositions appropriatefor clad fiber production.

The various oxide constituents can be partially replaced by thecorresponding metal halides in amounts such that the total halogencontent may be up to about 25% of the total halogen plus oxygen contentof the composition. Also, the ratio of R₂O to WO₃ is kept above 1:3,preferably 9:20. This avoids the reduction of W⁺⁶ to a lower oxidationstate that could introduce sources of attenuation in fibers.

The lightweight elements, hydrogen, boron and phosphorous, are desirableas oxide additives in the present glasses. This is because of theirfavorable influence on dopant ion lifetimes, particularly where erbiumis the dopant ion. As a matter of practical economics in opticalamplifier services, it is customary to pump erbium with a laseroperative at 980 nm. This poses a potential problem, since the laserpumps the ions to a level, commonly referred to as level 3, that isabove the actual fluorescing level.

The effective fluorescent radiation for amplification of a signal at 1.5μm emanates from an intermediate level known as level 2. The groundstate, to which ions return after they fluoresce, is referred to aslevel 1. Most effective amplification at 1.5 μm is achieved bymaximizing radiative output at intermediate level 2. To this end, it isdesirable to control the ion lifetime of both levels 2 and 3.

Tau-32, that is, the lifetime of an ion at level 3 before decaying tolevel 2, should be as short as possible. This short lifetime avoidsundesirable pump power losses due to upconversion. In contrast, tau-21,that is, the effective lifetime at level 2 before decaying to the groundstate (level 1), should be as long as possible.

In general, however, any means of shortening tau-32 also tends toshorten tau-21 as well. It has now been found that this general rule,for some reason not understood, does not hold true in the present,erbium-doped, alkali-metal-tungsten-tellurite glasses. This is shown inTABLE II, below.

In TABLE II, Example 6 is identical to Example 3 of TABLE I, that is,has no additive co-dopant. Examples 7-10 contain varying amounts ofPO_(2.5) as co-dopant, and Examples 11-14, likewise, contain varyingamounts of BO_(1.5) as co-dopant. The co-dopant, and the amount added ineach example, is shown in the second column as the oxide in cationpercent. In column 3, the fluorescence lifetime of level 3, that is,tau-32, is recorded for each composition in microseconds (μs). Likewise,column 4 records the fluorescence lifetime of level 2, that is, tau-21,for each composition in milliseconds (ms). The values in column 4 may bemultiplied by a thousand for direct comparison in microseconds.

TABLE II Example Co-Dopant Tau-32 (μs) Tau-21 (ms) 6 none 92 3.57 7 1PO_(2.5) 62 3.78 8 2 PO_(2.5) 37 3.69 9 4 PO_(2.5) 20 4.02 10 6 PO_(2.5)13 3.95 11 1 BO_(1.5) 78 4.06 12 2 BO_(1.5) 55 3.67 13 4 BO_(1.5) 463.59 14 6 BO_(1.5) 38 3.82

The uniquely different effect on tau-32 and tau-21 lifetimes exhibitedby the present glasses may also be seen in FIG. 3. There, the amount ofeither BO_(1.5) or PO_(2.5), added as co-dopant, is plotted on thehorizontal axis in cation percent. The tau-32 and tau-21 values areplotted on the vertical axis. Both values are plotted in microseconds(μs), employing a logarithmic scale, for direct comparison.

The essentially straight line, designated E, represents tau-21 for bothboron and phosphorous, co-doped compositions. The plotted points were soclose that drawing separate lines was not feasible. Straight lines,designated by the letters F and G, represent tau-32 for boron andphosphorous, co-doped compositions, respectively.

It will be observed that tau-21, shown by line E, remains essentiallyconstant, rather than decreasing, with increasing amounts of eitherco-dopant. At the same time, tau-32, shown by lines F and G, decreasesrapidly, as is desired, with increasing amounts of either co-dopant.

Tellurite glasses, having compositions as shown in TABLE I, were meltedby first mixing a batch in customary manner. The tellurium, tungsten andlanthanide components were introduced as oxides. The alkali metal oxideswere introduced as either the nitrate or the carbonate. The batch wasmanually mixed and placed in a 96% silica, or a gold, crucible. Thecrucible was introduced into an electric furnace operating at 750° C. tomelt the batch. Melting time was on the order of 30-60 minutes. Theglass thus produced was annealed at a temperature near the transitiontemperature (T_(g)) of the glass.

We claim:
 1. A family of glasses that have a thermal stability index(T_(x)−T_(g)) of at least 100° C., that have an emission spectrum with aFWHM bandwidth of at least 40 nm., and that have compositions consistingessentially of, as calculated in mole percent, 15-85% TeO₂, 5-55% WO₃,0.5-40% R₂O where Na, K, or mixtures, and 0.005-10% of an oxide of arare earth metal ion in the lanthanide series that is capable offluorescing when pumped with appropriate energy.
 2. A family of glassesin accordance with claim 1 wherein the oxide in the lanthanide series iserbium oxide or thulium oxide.
 3. A family of glasses in accordance withclaim 1 containing one or more optional, modifying components of MOwherein M is one or more of Ca, Sr, Ba, Mg, Zn, Cd, Pb, Y, Sb, La, Gd,Lu, Ti, Zr, Hf, Nb, Ta, Bi, H, B and P.
 4. A family of glasses inaccordance with claim 3 containing at least one optional, modifyingcomponent selected from 0-30% MO where M is an oxide of Mg, Ca, Sr, Ba,Zn, Cd and/or Pb, 0-20% Y₂O₃ and/or Sb₂O₃ and 0-15% TiO₂, Nb₂O₃ and/orTa₂O₅.
 5. A family of glasses in accordance with claim 3 wherein aportion of the metal oxides are replaced by metal halides providing thatthe atomic ratio of the total halide content to the total halide contentplus oxygen content of the glass is not over 1:4.
 6. A family of glassesin accordance with claim 1 wherein the ratio of total R₂O:WO₃ is atleast about 1:3.
 7. A family of glasses in accordance with claim 3containing a lightweight element selected from B, H, P and mixtures toshorten the tau-32 lifetime of a fluorescing ion.